1. Field of the Invention
This invention relates to a heater element suitable for use in a tube connecting device, wherein a set of tubes are melted and cut to be connected with each other by heating under a sterilized condition.
2. Description of the Related Art
When a dialysate bag and/or a waste liquid bag are changed in a continuous ambulatory peritoneal dialysis (CAPD), for example, or when tubes are connected to a blood-collecting bag and/or a blood component bag in a blood transfusion system, it is necessary to connect the tubes with each other in a sterilized condition.
There has been disclosed a tube connecting device for use in the situation like above, which connects a set of tubes with each other by heating and melting ends of the tubes in a sterilized condition (see U.S. Pat. No. 4,507,119).
The disclosed tube connecting device comprises a set of blocks capable of holding in parallel two tubes to be connected with each other, and a wafer (plate-shaped heater element) movably disposed between the blocks so that it can transversely cross the tubes. The wafer is heated while the two tubes are held by the blocks in parallel and in opposing directions with each other. The heated wafer is moved to melt and cut the tubes. Then, one of the blocks is moved relative to the other block in a direction perpendicular to the tube until the axes of the two tubes meet together in a line, and the wafer is removed. The melted ends of the tubes are fused together.
The wafer employed in the tube connecting device of this type is provided with a resistor disposed between folded metal plates having insulation layers (adhesive layers). When the resistor is fed with electricity, it generates heat so that the entirety of the metal plate is heated (see U.S. Pat. No. 4,501,951).
The resistor of the wafer is formed by etching a stainless foil or a nickel-chromium foil, which is accompanied by the following disadvantage.
(1) The stainless foil or the like used there is manufactured by a thin plate rolling process and has the thickness of about 25 .mu.m, which usually shows an appreciable variation. The width of the resistor also varies due to the limitations of the etching process. Thus, the resistance of the resistor cannot be stably obtained, namely, an error within .+-.5% in the designed resistance (eg 10.5.OMEGA.) can hardly be achieved. PA1 (2) It is difficult to adjust the resistance of the resistor, since the resistor is formed of stainless foil or the like having the predetermined thickness and excessive etching can occur. PA1 (3) The resistor made of stainless foil or the like is hard to adhere to the insulation layer of the wafer, since air is apt to enter into the gap between the resistor and the insulation layer. Hence, a portion of the resistor is often peeled off during manufacture of the wafer and the resistance of the resistor is caused to vary. When the resistor is in use, an expansion in the wafer thickness occur sometimes due to a temperature difference across the wafer and an increase in plate thickness. As a result, an imperfect fusion of the tubes is effected and leakage of the collected blood may occur by the imperfectly-connected tubes. PA1 (4) Manufacturing the resistor by etching process requires a number of manufacturing steps, a long manufacturing time and high manufacturing costs.
Once the wafer has been used, it may preferably be discarded because the surface of the wafer is contaminated with melted resin of the tubes or blood contained in the tubes. For this purpose, an invention has been devised thereby it becomes possible to judge if the wafer is not new (see U.S. Pat. No. 4,647,756).
This wafer, having the above-described function, comprises a bypass fuse which is electrically connected between both terminals of the resistor. When the wafer is new, the voltage applied across the both terminals of the resistor causes the current to bypasses the resistor and flow in the fuse. After the fuse has been burnt out, the current flows in the resistor to generate heat. When the wafer is not new, the fuse is already gone. Therefore, when the voltage is applied across both terminals, the current flows in the resistor directly. It can be determined, based on the difference between rise patterns of the applied voltages, whether the wafer is new or old. In this arrangement, an additional manufacturing step to connect the fuse is required after the resistor has been formed by etching, which however, brings an increase in the number of manufacturing steps, the manufacturing time and the manufacturing costs.